1. Field of the Invention
The invention relates to a monitoring device for vehicles, in particular, motor vehicles, comprising a mirror glass behind which at least one camera is arranged.
2. Description of the Related Art
Interior rearview mirrors of motor vehicles are known which are provided with a sensor. In order for the sensor to fulfill its function, the mirror glass is provided with a cutout. However, cutouts in the reflective layer of mirror glass are generally not acceptable for esthetic reasons. Often, it is also desired that the sensor be invisible. Moreover, there are also regulations in the automobile industry which concern the geometric size of the mirror surface.
It is an object of the present invention to configure the monitoring device of the aforementioned kind such that the camera is not visible behind the mirror glass, but such that the mirror glass still can fulfill its function reliably.
In accordance with the present invention, this is achieved in that the mirror glass comprises a reflective layer which is reflective within the visible spectral range of light and behind which the camera is arranged.
In the monitoring device according to the invention, the camera is positioned behind the reflective layer which is designed to reflect preferably up to 50% within the visible spectral range of light. Accordingly, the camera is not visible behind the mirror glass while the mirror glass has the desired high reflection. When the mirror glass is, for example, used as an interior rearview mirror of a motor vehicle, the reflective layer has a reflective spectrum for reflecting light with the minimum percentage required pursuant to the regulations for automotive mirrors. Advantageously, the camera is a CCD (charge coupled device) camera or CMOS (complementary metal oxide semiconductor) camera. The mirror glass with the reflective layer is sufficiently transparent within the near infrared range that video images can be taken with such a camera. In the case that the illumination is not sufficient, it is possible to provide additional illumination by means of an auxiliary lighting unit, preferably in the form of LEDs (light-emitting diodes) which emit a wavelength for which the reflective layer is sufficiently transparent and for which the camera has a satisfactory sensitivity. In the case of bad lighting situations within the visible range, the required area can be additionally illuminated, undetected by the human eye, by moving, for example, the transparent spectral range of the mirror glass along the wavelength axis such that the transmission reaches a maximum value and covers at the same time the sensitivity maximum of the CCD or CMOS camera. The auxiliary lighting unit is advantageously realized by infrared LEDs which emit a light in the wavelength range which is within the transmission range of the reflective layer and for which the camera has the required sensitivity.
The camera is advantageously connected to the electric power supply of the motor vehicle. The video images taken by it can be shown on a monitor. However, it is also possible to store the video images on a storage medium, for example, a video tape, a hard disk etc. It is also possible to store the image and simultaneously show it on a monitor.